1. Field of the Invention
The present invention relates to a method for preparing a highly dispersed supported platinum catalyst, and more particularly to a method for preparing a highly dispersed supported platinum catalyst in which a reducing agent prepared by mixing ethylene glycol and sodium borohydride is added to a mixture of a platinum precursor and a carbon support.
2. Background
A fuel cell is a power generating unit converting chemical energy of a fuel into electrical energy. The fuel cell is an alternative energy having an excellent efficiency as compared with a conventional internal combustion engine and rarely discharging contamination material. The fuel cell can be used to a wide range of applications including a computer, a camcorder, and a mobile phone, a power source of a vehicle, a train, a ship, or a submarine, a power supplying and heating unit for home, and a small or middle-sized power plant. Recently, the fuel cell has been intensively developed as a power source of a vehicle and also has been concerned as a power source of a mobile phone.
The fuel cell includes an oxidation electrode, a reduction electrode, and an electrolyte and generates electrical energy by using the catalyst reaction of the oxidation and the reduction electrodes and penetration of ions in the electrolyte. Hydrogen ions generated by oxidizing the fuel injected in the oxidation electrode are introduced into an electrolyte membrane. In the reduction electrode, the hydrogen ions supplied from the electrolyte membrane, the oxygen injected to the reduction electrode, and electrons supplied from the oxidation electrode are reacted to produce water. A metal having an excellent electrochemical reaction activity such as platinum is used as an electrode catalyst to generate the electrode reactions at a low temperature. However, although platinum is a single catalyst capable of expediting oxidation of a fuel (hydrogen or alcohol) and reduction of oxygen to near 100° C. from a normal temperature, since the platinum should be buried as less as possible and the price of platinum is high, it is important to maximize the activity per unit weight. In order to accomplish this, the reaction activity area should be maximized by regulating the size of particles in the range of nano sizes. If the thickness of the catalyst layer increases in the fuel cell, the diffusion resistance of a reactant or a product increases, thereby decreasing the overall efficiency of the fuel cell. Therefore, a highly supported catalyst containing larger amount of platinum as compared with a support body capable of reducing the thickness of the catalyst layer is necessary. But as the amount of contained platinum increases, the sizes of platinum particles increase, reducing the reactant activity area. To solve this problem, a variety of methods have been developed.
For example, an initial impregnation method has been frequently used to prepare platinum catalysts. But this method has a drawback that when the amount of the supported metal becomes larger, the average size of the platinum catalyst particles becomes larger and the size distribution thereof becomes widened.
U.S. Pat. No. 4,186,110 suggests a precipitation method in which platinum is reduced in liquid phase by transforming a carbon support body and slurry of water to a base, injecting a platinum chloride acid solution, and adding a reducing agent such as formaldehyde. Further, U.S. Pat. No. 5,068,161 suggests a method for preparing a catalyst of up to 60 wt % supported platinum with respect to the entire catalyst weight using the method.
Such precipitation method, however, produces a catalyst with nonuniform distribution of platinum particles, although it is simple and suitable for mass production.
A colloid method has been suggested, in which minute platinum particles are prepared in an aqueous solution or an organic solution, and the platinum particles are adsorbed to a carbon support body. The preparation process, however, is complex and the sizes of platinum particles produced are varied depending on the factors such as pH and temperature. In addition, the time for adsorption is relatively long.
Further, Korean Patent Laid-open Publication No. 2004-88288 suggests a platinum/support body catalyst transformed into a base by adding a pH regulating material to support platinum, the density of the pH regulating material is not uniform partially and the platinum particles are not uniformly distributed, which prevents platinum catalyst from being prepared with a high reproducibility.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.
Accordingly, there is thus a need for a method for preparing a highly dispersed supported platinum catalyst having an improved platinum particle sizes and distribution.